There is an increasing interest on the use of hydrocarbon components of biological origin from renewable sources in fuels to replace the fossil starting materials. There are various literature references relating to production of fuel compositions from biological starting materials. For example Finnish Patent 100248 describes a production of middle runnings (middle distillate) of vegetable oil. In the first stage of the production method the fatty acids or triglycerides of vegetable oil are hydrogenated to n-paraffins in a HDO/HDS step and in the second stage said n-paraffins are catalytically converted into branched-chain paraffins (isomerisation). However, this process has the drawback that it needs a purification step between the two process steps since the isomerisation catalyst is poisoned by the sulphur compounds that are formed in the first stage. Additionally, yield of the desired product decreases, as the number of process steps increases. For example, when two process steps each operating with an 80% yield rate are arranged in series, the final yield rate after both process steps will be only 64%. Further, the two step process adds investment costs, process complexity, intermediate steps and operating costs.